Two-wire to four-wire conversion circuit for a data switching center

ABSTRACT

In a two-wire to four-wire data switching center, the usual hybrid transformer(s) is replaced by a single transformer with a symmetrical secondary. Interaction between both directions of transmission is avoided by connecting the extremities of the secondary respectively to the output of a first current amplifier and to the input of a second current amplifier. This circuit is intended specifically for use with electronic switching such as that using MOS switching crosspoints and stages.

United States Patent Colardelle et al.

[ 1 Sept. 5, 1972 [541 TWO-WIRE TO FOUR-WIRE CONVERSION CIRCUIT FOR A DATA SWITCHING CENTER [72] Inventors: Joel Serge Colardelle, Creteil; Pierre Girard, Paris; Claude Paul Henri Lenouge, Maurepas, all of France [73] Assignee: International Standard Electric Corporation, New York, NY.

[22] Filed: Oct. 12, 1970 [21] Appl. No.: 79,777

[30] Foreign Application Priority Data Oct. 17, 1969 France ..6935622 [52] US. Cl ..179/170 D, 179/170 T [51] Int. Cl. ..H04b 1/58 [58] Field of Search...179/81 A, 170 D, 170 NC, 170

T, l79/170.2; 330/188, 30 R; 333/11, 25

[56] References Cited UNITED STATES PATENTS Elliott ..179/ 170 T 3,540,049 1 H1970 Gaunt ..179/170 NC 3,108,157 10/1963 Feiner ..333/11 2,733,304 l/l 956 Koenig ..179/170 T FOREIGN PATENTS OR APPLICATIONS 214,818 5/1958 Australia 1 79/170 T Primary Examiner-Katl1leen H. Clalfy Assistant Examiner-William A. Halvestine Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson, Jr., James B. Raden, Delbert P. Warner and Marvin M. Chaban [57] ABSTRACT In a two-wire to four-wire data switching center, the usual hybrid transformer(s) is replaced by a single transformer with a symmetrical secondary. Interaction between both directions of transmission is avoided by connecting the extremities of the secondary respectively to the output of a first current amplifier and to the input of a second current amplifier. This circuit is intended specifically for use with electronic switching such as that using MOS switching crosspoints and stages.

2 Claims, 4 Drawing Figures SHEET 1 UF 2 PATENTEDSEP 5 I912 mm 6 mm {1111 7 m m J m 2[ ER d um i m 6 w m 00 N w w 56 L u 4 u u u n llll C m 1|! n u h N w n M 0 H I Paw I l IIL "Ill I l l l I I I|L M 4 R 3 Q, R T /H U M l [w x7 9. d 4 ma wfl MW A Home y TWO-WIRE TO FOUR-WIRE CONVERSION CIRCUIT FOR A DATA SWITCHING CENTER The present invention concerns a two-wire to fourwire conversion circuit for coupling two-wire lines to a four-wire data switching network.

It is known that transmission between subscribers (or concentrators) and a data switching center, such as a telephone central exchange, is carried out generally over two-wire lines in order to save copper.

It is known also that if the switching in the central exchange involves the use of electroniccrosspoints, it is generally necessary that the two transmission directions be separated, so that four-wire circuits are needed. Four-wire switching networks have been described in the following French patent application:

a. PCM Network: Patent Application Ser. No. 6,901,888 filed on Jan. 30, 1969.

b. Amplitude modulation network: Patent Application PV 156 405 filed on June 25, 1968.

In the prior art, two-wire to four-wire conversion has been carried out by means of bridge circuits in the form of hybrid transformers. Such conversion circuits use one or two transformers of hybrid type which must be of high quality and the balancing of the various impedances of the bridge must be very accurate in order to minimize the interaction between the two directions of transmission and the insertion loss.

In the present invention, two-wire to four-wire conversion is carried out by means of a transformer, the primary winding of which is connected to the line and the secondary winding of which is constituted by a center-tapped winding. One of the ends of the secondary winding is connected to the input of a current amplifier and the other end to the output of an amplifier of the same type. It will be remembered that an ideal current amplifier presents zero input impedance and an infinite output impedance. Such an ideal current transformer will be realized, to a fair approximation, by a bipolar transistor in common base configuration which presents a current gain slightly lower than unity. The circuit according to the invention enables the elimination of high accuracy balancing resistors and the use of current amplifiers enables the elimination of insertion losses due to the transforemers and to the switching network, even if this latter presents an appreciable resistance. This is the case when the crosspoints are achieved by means of MOS transistors such as those described in the patent application referenced b).

The object of the present invention is thus to design a two-wire to four-wire connection circuit for coupling two-wire transmission lines to a four-wire data switching network.

According to the invention there are provided means associated with each line including a transformer with a center tapped symmetrical secondary winding whose tap is grounded through a resistor R1. Means are provided for connecting one end of the secondary to the input of a first current amplifier and the other end of said winding to the output of a second current amplifier. Means are provided for connecting the output of the first amplifier to a receiver. Means are provided for injecting into the input of the second amplifier a current proportional to the voltage supplied by a transmitter so that the energy received over the line is distributed between the receiver and the resistor R1 and the energy transmitted by the transmitter is distributed between the line and the resistor R1.

According to another characteristic of the invention there are provided means located in a junctor for connecting the output of the first amplifier of a given line to the input of a second amplifier of another line said means comprising, in particular, an amplifier connected to the output of the first amplifier which delivers a voltage the amplitude of which may be chosen in such a way as to cancel the insertion loss of the conversion circuits and of the switching network.

The above mentioned and other features and objects of this invention will become apparent by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 represents the principal diagram of the twowire to four-wire conversion circuit according to the invention;

FIG. 2 represents the detailed diagram of a four-wire path through which a link is set up between lines In and Lb;

FIG. 3 represents the unfolded diagram of a junctor; FIG. 4 represents the characteristic IE4 =f(VCE4) of the transistor Q4.

FIG. 1 represents the principal diagram of the circuit according to the invention for optimizing the adaptation of a two-wire line L of impedance ZL to a transmitter and to a receiver of impedances ZK and ZR. It comprises:

the symmetrical transformer T comprising a primary winding with p turns and a center-tapped secondary winding with 2p turns, this tap being grounded through the resistor R1. Each one of the extremities m and n of this winding constitutes, together with the common ground connection, a two-wire line for unidirectional data transmission; the current amplifiers Gm and Gn of very low input impedance Re which deliver a constant current of amplitude 12 a1 1, if one designates by 11 and 12 the input and output currents and by a the current gain. These amplifiers present an output impedance Rs which is very high. The receiver R and the transmitter K are symbolically shown, respectively, by an impedance Zr and a voltage source e of internal impedance ZK. These impedances ZR and ZK are chosen such as they are very high with respect to Re and very low with respect to Rs. It will now be shown that this circuit assures the twowire to four-wire conversion, i.e. v

The energy sent by the transmitter K is transmitted only towards the line L (transmission direction N);

The energy received over the line L is transmitted only towards the receiver R (transmission direction M).

1. ENERGY-SENT B Y THE TRANSMITTER K It will be assumed that the inverter W1 is in position b and that the switch W2 located over the wire m is open. The current Inl delivered by the transmitter K is injected in the amplifier Gn and its value is Inl (e/ZK+ Re) 5 e/ZK This amplifier supplies a current ln2 aInl which is injected in the circuit constituted by the series connection of the resistor R1 and by one half of the secondary winding of the-transformerT in which is reflected the impedance ZL of the line. The potential difference Vn Vg between the extremity of the transformer and the ground is thus Vn Vg (R1 ZL)-In2. If one sets R1 ZL ZK, it is seen that the energy is distributed equally between ZL and R1, and one has an auto-transformer, and

Vm- Ve= Ve- Vn=u..

From these equations, one may write (Ve Vg)+(Vm-' Ve)= Vm- Vg =+u -u'=0.

line is equally shared between the line L and the resistor RI and that no energy at all is directed toward the receiver R, this being true regardless of the value of ZK.

2. ENERGY RECEIVED OVER THE LINE L One may consider that this energy is supplied by a voltage source u (inverter W1 in the position a) of internal impedance ZL. This voltage a produces two voltages which are equal and of opposite signs in the two halves of the secondary winding.

The equivalent circuit of the extremity n comprises thus a voltage source u of internal impedance ZL +Rl which is loaded by the resistor Rs, the value of which is practically infinite. Almost the totality of the voltage u appears thus across the terminals of Rs but since, in practice, each one of the current amplifiers Gm and Gn is materialised by a bipolartransistor in common base configuration which presents a very low reverse voltage transfer ratio (about 5.10 no signal at all appears practically at the output of the amplifier Gn.

The equivalent circuit of the extremity m comprises a voltage source u of internal impedance ZL R1 which is loaded by a very low resistor Re, so that one has:

If ZL R1, it is seen that half of the energy transmitted in the direction M (from line receiver) is dissipated in the resistor R1.

Last, one has [m2 =au/2Rl 2 FIG. 2 represents the detailed diagram of the fourwire path through which the link is set up between the lines La and Lb through a junctor Jx. The path between a line and the junctor comprises:

The line circuit LCa (or LCb) The part Jxa (or Jxb) of the junctor Jx The capacitors C2 and C3 which assure DC level isolation between the parts Jxa and Jxb of the junctor;

The resistors Rds interposed over the wires of each one of the transmission directions M and N which symbolize the path set up by means of MOS transistors, this network being that described in the patent application referenced b. Each one of these resistors Rds represents the sum of the drainsource resistances of the MOS transistors located over the path, and one may set Rds 60O ohms. It will be noted that this value may vary according to the individual. characteristics of the MOS transistors.

A line circuit, such as LCa, comprises the trans- I former Ta identical to the transformer T of FIG. 1, the

resistors Rla, Rlbsuch as Rla =R1b 2R1, andthe bipolar NPN transistors Qla, 01b in common base configurationv which materialize, as has been seen hereabove, the current amplifiers Gm and Gn.

The part Jxa (or Jxb) of a junctor comprises For the transmission direction M the complementary bipolar transistors Q3a, 04a and the resistors R3a, R4a;

For the transmission direction N the NPN transistor 05a and the resistors RSa, R6a.

One will now study the transmission of energy from the line In towards the line Lb using the diagram of FIG. 3 which also may be considered as an unfolded representation of the circuits of one of the parts Jxa or Jxb of the junctor Jx.

If one considers a link set up between the lines La and Lb (FIG. 2), the path used for each one of the transmission directions uses the circuits represented on this Figure. In order to simplify the Figure and the description, the letters a and b associated with the transistors have been omitted.

This junctor Jx receives, in the transmission direction M, a current Im2. If one assumes that all the transistors represented on the Figure are biased in Class A, one may write, for the small signals analysis:

Vb4 base voltage of the transistor Q4 Ve4 emitter voltage of this transistor. one has Vb4 Ve4 aR3'Im2 This voltage is applied to the transistor Q5 through an impedance-R6 providing a current I0 Ve4/R6 =a-R3/R6 -Im2. The collector current of this transistor is Inl =a-I0 =a -R3lR6- Im2.

It is injected in the base of the transistor Q2, the collector current of which is In2 =a R3/R6 -Im2 3 As it has been seen during the study of FIG. 1, the voltage applied to the primary of the transformer T is u=(2R1.Im2/a (2 and the voltage collected on the primary of the trans- .f me i u R1-In2 l By replacing In2 and [m2 in the equation (3) by their s lnes given in these equations l and (2), one obtains:

It is seen then that the transmission loss is independant of the value of the resistances Rds. It has also been seen, during the study of FIG. 1, that the incoming lines (La) and the outgoing lines (Lb) were loaded by the same impedance R1 ZL so that the ratio between the input and output powers is equal to the square of the voltage ratio given by the equation (4).

It will be noted that one may obtain a zero transmission loss in the switching network of FIG. 2 by choosing :a R3/2R6= 1.

The DC operation of these circuits will be now studied by using the values of supply voltages indicated in FIG. 2 and by setting, for reasons of simplification R6 600 ohms,

The voltage base-emitter drop of a bipolar transistor is of 1 volt,

One will neglect a, which has a value of about 1.

1. Direction of Transmission M.

The emitter current of O1 is IE1 =(7/1,2) 5,8 mA.

The collector voltage of Q3 is VC3 12 (5,8 1,2) 5 v.

The emitter voltage of O4 is thus VE4 6 v, and its emitter current is IE4 (6/1,2) 5 mA. These values VB and IE4 determine the point of static operation of the transistor.

2. Direction of Transmission N The emitter current of O5 is IE5 (7/1 ,2) 5,8 mA. The emitter and collector currents of Q2 have the same value. It is thus seen that all the transistors in common base configuration are biased at 5,8 rnA so that they operate as Class A linear amplifiers for signals having a peak to peak amplitude which ranges between and 10 mA.

We shall now study the dynamic operation, for large signals, of the common collector stage comprising the transistor Q4.

FIG. 4 represents the diagram IE4 =f( VCE4), VCE4 being the collector-emitter voltage of this stage, the static load resistance of which (in DC) has a value RLs R4 1,200 ohms. The operation point P has the coordinates 6 v, mA as it has been seen hereabove. The dynamic load RLd is constituted by the parallel connection of the resistors R4 and R6, vizus RLd 400 ohms. It is seen on this FIG. 4 that the dynamic range of the stage is mA peak to peak, in the common base stages.

The dynamic range for large signals in the path connecting the lines La and Lb is thus of 10 rnA, vizus 6 volts peak to peak for R1 SL 600 ohms.

While the principles of the above invention have been described in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made by way of example and not as a limitation of the scope of the invention.

We claim:

1. A two-wire to four-wire conversion circuit for coupling a two-wire data transmission line to a receiver and to a transmitter while providing electrical isolation between these two elements regardless of the values of their impedances comprising a transformer having a primary winding with p turns connected to a line of impedance (ZL) and a symmetrical center tapped secondary winding with 2p turns, means connecting the center tap .to ground through a resistor (R1), a first amplifier having an input which is connected to a first terminal of the secondary and an output connected to a receiver of internal impedance (ZR), a second amplifier having an input which is connected to a transmitter (K) and an output connected to the second terminal of the secondary winding, said transmitter including an internal impedance (ZK) coupled to a voltage source e, said first and second amplifiers each including current amplifiers of gain a where the gain is less than unity and the amplifiers have a low input impedance Re relative to output impedances (RS), each of said current amplifiers being formed to include a transistor in common base configuration, whereby an AC. voltage u applied to the primary of the transformer produces, at the output of the first amplifier a current 1M2 (om/2R1) while no energy at all is dissipated in the impedance (ZK), and a voltage e applied to the second amplifier through the impedance (ZK) produces an input current Inl e/Rl (where ZK R1) and produces across the primary of the transformer a voltage u R1 'ln2 rate while no energy at all is dissipated in the impedance ZR, these values of (Im2) and u being given for the case where R1 ZL ZR and the current (M2) is the output current of the second amplifier.

2. A two-wire to four-wire conversion circuit for coupling a two-wire data transmission line to a fourwire junctor while providing electrical isolation between signals transmitted in opposite directions in the fourwire junctor regardless of the values of impedances in the circuit, comprising a transformer having a primary winding with p turns connected to a line of impedance (ZL) and a symmetrical center tapped secondary winding with 2p turns, means connecting the center tap to ground through a resistor (R1) thereby representing the connection to ground of two wires of said four-wire junctor, a first amplifier having an input which is connected to a first terminal of the secondary and an output connected to a third wire of the fourwire junctor, said third wire having an impedance (ZR), a second amplifier having an input which is connected to a fourth wire of said 4-wire junctor and an output connected to the second terminal of the secondary winding, said fourth wire including an impedance (ZK) coupled to a voltage source e, said first and second amplifiers each including current amplifiers of gain a where the gain is less than unity and the amplifiers have low input impedances (Re) relative to output impedances (RS), each of said current amplifiers being formed to include a transistor in common base configuration, whereby an AC. voltage u applied to the primary of the transformer produces, at the output of the first amplifier a current Im2 (om/2R1) while no energy at all is dissipated in the impedance (ZK), and a voltage e applied to the second amplifier through the impedance (ZK) produces an input current Inl e/Rl (where ZK R1) and produces across the primary of the transformer a voltage u Rl'In2 me while no energy at all is dissipated in the impedance ZR, these values of (Im2) and u being given for the casewj ere R1=ZLXZR and the current (M2) is the output current of the second amplifier. 

1. A two-wire to four-wire conversion circuit for coupling a two-wire data transmission line to a receiver and to a transmitter while providing electrical isolation between these two elements regardless of the values of their impedances comprising a transformer having a primary winding with p turns connected to a line of impedance (ZL) and a symmetrical center tapped secondary winding with 2p turns, means connecting the center tap to ground through a resistor (R1), a first amplifier having an input which is connected to a first terminal of the secondary and an output connected to a receiver of internal impedance (ZR), a second amplifier having an input which is connected to a transmitter (K) and an output connected to the second terminal of the secondary winding, said transmitter including an internal impedance (ZK) coupled to a voltage source e, said first and second amplifiers each including current amplifiers of gain Alpha where the gain is less than unity and the amplifiers have a low input impedance Re relative to output impedances (RS), each of said current amplifiers being formed to include a transistor in common base configuration, whereby an A.C. voltage u applied to the primary of the transformer produces, at the output of the first amplifier a current IM2 ( Alpha u/2R1) while no energy at all is dissipated in the impedance (ZK), and a voltage e applied to the second amplifier through the impedance (ZK) produces an input current In1 e/R1 (where ZK R1) and produces across the primary of the transformer a voltage u'' R1.In2 Alpha e while no energy at all is dissipated in the impedance ZR, these values of (I m2) and u'' being given for the case where R1 ZL ZR and the current (In2) is the output current of the second amplifier.
 2. A two-wire to four-wire conversion circuit for coupling a two-wire data transmission line to a four-wire junctor while providing electrical isolation between signals transmitted in opposite directions in the four-wire junctor regardless of the values of impedances in the circuit, comprising a transformer having a primary winding with p turns connected to a line of impedance (ZL) and a symmetrical center tapped secondary winding with 2p turns, means connecting the center tap to ground through a resistor (R1) thereby representing the connection to ground of two wires of said four-wire junctor, a first amplifier having an input which is connected to a first terminal of the secondary and an output connected to a third wire of the four-wire junctor, said third wire having an impedance (ZR), a second amplifier having an input which is connected to a fourth wire of said 4-wire junctor and an output connected to the second terminal of the secondary winding, said fourth wire including an impedance (ZK) coupled to a voltage source e, said first and second amplifiers each including current amplifiers of gain Alpha where the gain is less than unity and the amplifiers have low input impedances (Re) relative to output impedances (RS), each of said current amplifiers being formed to include a transistor in common base configuration, whereby an A.C. voltage u applied to the primary of the transformer produces, at the output of the first amplifier a current Im2 ( Alpha u/2R1) while no energy at all is dissipated in the impedance (ZK), and a voltage e applied to the second amplifier through the impedance (ZK) produces an input current In1 e/R1 (where ZK R1) and produces across the primary of the transformer a voltage u'' R1.In2 Alpha e while no energy at all is dissipated in the impedance ZR, these values of (Im2) and u'' being given for the case where R1 ZL ZR and the current (In2) is the output current of the second amplifier. 